The invention relates to a method of deriving signals at frame intervals from a received television signal which has two predetermined digital words which alternate line-by-line for a substantial part of a frame period of the television signal but which are each repeated in a respective pair of adjacent lines in the remaining part of the frame period, said method comprising comparing portions of each line period with one of the digital words and producing therefrom an error signal representing the number of bits in said portions which differ from said digital word and using said error signal to produce signals at frame intervals. The invention also relates to apparatus for use with this method.
The European Broadcasting Union documents "Television Standards for the Broadcasting Satellite Service Specification of the C-MAC/Packet System" SPB 284, 3rd revised edition dated December 1984, and "Methods of Conveying C-MAC/Packet Signals in Small and Large Community Antenna and Cable Network Installations" SPB 352, dated December 1984 describe three forms of television signal which are referred to as C-MAC, D-MAC and D2-MAC. For synchronising purposes each line of these signals carries one of the two digital sync. words W1 or W2 where W1 is of the form 001011 and W2 is the inverse, namely 110100. For a system having a frame comprising 625 lines, the sync. words alternate line-by-line for lines 1 to 621 whilst lines 622 and 623 both convey one sync. word whilst lines 624 and 625 both convey the other sync. word, this change in sync. word pattern indicating the end of a frame. For an odd frame line 1, and hence line 621 will convey the W2 sync. word whilst lines 622 and 623 will convey the W1 sync. word and lines 624 and 625 the W2 sync. word. This situation is the inverse for even frames such that line 1 conveys the W1 sync. word as will lines 624 and 625 whilst lines 622 and 623 will convey the W2 sync. word. PCT Patent Application No. WO84/02242 describes apparatus for obtaining line and frame synchronising signals from alternating digital sync. words but this is based on an earlier version of a specification for a C-MAC television signal where the two sync. words each had 7 bits whilst the pairs of lines using the same sync. words straddled the boundary between odd and even frames.